[RISCV] Remove overly restrictive assert from negateFMAOpcode.

[craig.topper]

Summary

It's possible that both multiplicands are being negated. This won't
change the opcode, but we can delete the two negates. Allow this
case to get through negateFMAOpcode.

I think D152260 will also fix this test case, but in the future
it may be possible for an fneg to appear after we've already converted
to RISCVISD opcodes in which case D152260 won't help.

[fakepaper56]

LGTM.

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

 //===-- RISCVISelLowering.cpp - RISC-V DAG Lowering Implementation  -------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
static SDValue performBITREVERSECombine(SDNode *N, SelectionDAG &DAG,
   SDLoc DL(N);
   return DAG.getNode(RISCVISD::BREV8, DL, VT, Src.getOperand(0));
 }
 
 // Convert from one FMA opcode to another based on whether we are negating the
 // multiply result and/or the accumulator.
 // NOTE: Only supports RVV operations with VL.
 static unsigned negateFMAOpcode(unsigned Opcode, bool NegMul, bool NegAcc) {
-  assert((NegMul || NegAcc) && "Not negating anything?");
-
   // Negating the multiply result changes ADD<->SUB and toggles 'N'.
   if (NegMul) {
     // clang-format off
     switch (Opcode) {
     default: llvm_unreachable("Unexpected opcode");
     case RISCVISD::VFMADD_VL:  Opcode = RISCVISD::VFNMSUB_VL; break;
     case RISCVISD::VFNMSUB_VL: Opcode = RISCVISD::VFMADD_VL;  break;
     case RISCVISD::VFNMADD_VL: Opcode = RISCVISD::VFMSUB_VL;  break;

llvm/test/CodeGen/RISCV/rvv/vfma-vp.ll

; CHECK-NEXT:    ret
   %elt.head = insertelement <vscale x 8 x double> poison, double %b, i32 0
   %vb = shufflevector <vscale x 8 x double> %elt.head, <vscale x 8 x double> poison, <vscale x 8 x i32> zeroinitializer
   %head = insertelement <vscale x 8 x i1> poison, i1 true, i32 0
   %m = shufflevector <vscale x 8 x i1> %head, <vscale x 8 x i1> poison, <vscale x 8 x i32> zeroinitializer
   %negvb = call <vscale x 8 x double> @llvm.vp.fneg.nxv8f64(<vscale x 8 x double> %vb, <vscale x 8 x i1> %m, i32 %evl)
   %v = call <vscale x 8 x double> @llvm.vp.fma.nxv8f64(<vscale x 8 x double> %negvb, <vscale x 8 x double> %va, <vscale x 8 x double> %vc, <vscale x 8 x i1> %m, i32 %evl)
   ret <vscale x 8 x double> %v
 }
+
+define <vscale x 1 x half> @vfma_vv_nxv1f16_double_neg(<vscale x 1 x half> %a, <vscale x 1 x half> %b, <vscale x 1 x half> %c, <vscale x 1 x i1> %m, i32 zeroext %evl) {
+; CHECK-LABEL: vfma_vv_nxv1f16_double_neg:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    vsetvli zero, a0, e16, mf4, ta, ma
+; CHECK-NEXT:    vfmadd.vv v9, v8, v10, v0.t
+; CHECK-NEXT:    vmv1r.v v8, v9
+; CHECK-NEXT:    ret
+  %nega = call <vscale x 1 x half> @llvm.vp.fneg.nxv1f16(<vscale x 1 x half> %a, <vscale x 1 x i1> %m, i32 %evl)
+  %negb = call <vscale x 1 x half> @llvm.vp.fneg.nxv1f16(<vscale x 1 x half> %b, <vscale x 1 x i1> %m, i32 %evl)
+  %v = call <vscale x 1 x half> @llvm.vp.fma.nxv1f16(<vscale x 1 x half> %nega, <vscale x 1 x half> %negb, <vscale x 1 x half> %c, <vscale x 1 x i1> %m, i32 %evl)
+  ret <vscale x 1 x half> %v
+}